The repair of mismatched bases in DNA is probably universal in living organisms yet little is known about the specificity of such repair. In addition to its role in recombination, post-synthetic mismatch repair in Echerichia coli is thought to correct replication errors. We have proposed that this correction system is the major one for removal of potential frameshift mutations. The present proposal seeks to determine the specificity of repair using Escherichia coli and bacteriophage lambda. Two approaches will be used. First, cI mutations in lambda will be generated from strains deficient in mismatch repair. These strains show high mutation rates because base pair mismatches arising as replication errors are not corrected. The cI mutations will be defined by genetic mapping and DNA sequence analysis. Heteroduplexes containing wild type and mutant cI alleles will be constructed and scored for repair after transfection in E.coli. This will define which of the two possible mismatches (or both) leading to a base pair change is important. In addition to the wild type and mismatch repair deficient mutants we will also determine mutational specificity in mutD strains which are defective in polymerase III associated proof reading of errors. In such a strain frameshift mutations should not be prominent. The second approach involves constructing heteroduplexes using known base pair substitution mutations in all possible combinations, followed by transfection and scoring for repair. Additionally heteroduplexes derived from frameshift, deletion or insertion mutations will also be constructed and tested for repair. This information may increase our understanding of the mechanism of genetic diversity and the nature of spontaneous mutation which could potentially affect any process in living cells.